Electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction of the heart, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm. Electrodes have also been used to sense and deliver pacing pulses to the atrium and ventricle. The electrode in the atrium senses the electrical signals that trigger the heartbeat. The electrode detects abnormally slow (bradycardia) or abnormally fast (tachycardia) heartbeats. In response to the sensed bradycardia or tachycardia condition, a pulse generator produces pulses or signals to correct the condition. The same node used to sense the condition is also used in the process of delivering a corrective pulse or signal from the pulse generator of the pacemaker.
There are four main types of pulses which are delivered by a pulse generator. Two of the signals or pulses are for pacing the heart. First of all, there is a pulse for pacing the heart when it is beating too slowly, and the pulse triggers the heart beat. The pulses are delivered at a rate to increase the heart rate to a desired level. The second type of pacing, called antitachycardia pacing, is used on a heart that is beating too fast. In antitachycardia pacing, the pacing pulses are delivered initially at a rate faster than the beating heart. The rate of the pulses is then slowed until the heart rate is at a desired level. The third and fourth type of pulses are used when the heart is beating too fast and the heart is fibrillating. The third type is called cardioversion. This is delivery of a relatively low energy shock, typically in the range of 0.75 to 1 joule, to the heart. The fourth type of pulse or signal is a defibrillation signal which is the delivery of a high energy shock, typically up to 34 joules, to the heart.
Sick sinus syndrome and symptomatic AV block constitute the major reasons for insertion of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by electrodes implanted directly onto the epicardium, where transvenous pacing may be temporary or permanent. In temporary transvenous pacing, an electrode lead is introduced into a peripheral vein and fluoroscopically positioned against the endocardium. The external terminals of the leads are connected to an external cardiac pacemaker which has an adjustable rate and milliamperage control. Temporary transvenous pacing is utilized prior to insertion of a permanent pacing system and in situations in which the indication for pacing is judged to be reversible (drug-induced AV block or bradycardia) or possibly irreversible and progressive (AV and bundle branch blocks associated with myocardial infarction).
Permanent transvenous pacing systems are implanted under sterile surgical conditions. An electrode lead is generally positioned in the right ventricle and/or in the right atrium through a subclavian vein, and the proximal electrode terminals are attached to a pacemaker which is implanted subcutaneously. Some patients require a pacing system to correct an abnormally slow heart (bradycardia condition) as well as a defibrillation system to detect when the heart starts beating abnormally fast (tachycardia condition) and to defibrillate or deliver a pulse to the heart to correct the abnormally fast heartbeat. In the past, a common practice for a patient having both of these conditions would be to provide two different leads attached to the heart. One would be implanted for delivering pacing signals to the heart to correct for the bradycardia condition. A separate lead would be implanted to sense a fast beating heart and defibrillate the heart to correct for the tachycardia condition. One lead is placed in the atrium and the other lead is placed in the ventricle.
Having two separate leads implanted within the heart is undesirable for many reasons. Among the many reasons are that the implantation procedure for implanting two leads is more complex and also takes a longer time when compared to the complexity and time needed to implant a single lead. In addition, two leads may interact with one another after implantation or in vivo which can result in dislodgment of one or both of the leads. In vivo interaction may also cause abrasion of the insulative layer along the lead which can result in an electrical failure of one or both of the leads. Another problem is that as more leads are implanted in the heart, it can become increasingly difficult to add additional leads. Two separate leads also increase the risk of infection and may result in additional health care costs associated with implantation and follow-up.
Because of these problems, a single lead, called a single pass lead design, having electrodes for both pacing and sensing in both chambers of the heart has been used. One drawback of some current single pass lead designs is that they use "floating" electrodes or electrodes which are not attached to the endocardial wall of the heart. The floating electrodes lay in the blood pool or against the endocardial wall of the heart and the electrode may move slightly within the heart. The electrode positioned within the atrium of a single-pass endocardial lead generally is an electrically conductive cylindrical ring or semicylindrical ring structure, which does not allow for tissue ingrowth into the electrode. Since the location of the electrodes is not fixed with respect to the atrial wall, the electrical performance of these electrodes varies and is generally less than optimal. Both the electrical sensing capability as well as the pacing delivery capability of such electrodes are suboptimal. The pacing parameters of such a floating electrode are also suboptimal. In addition, the floating electrodes can require increased voltage which unnecessarily drains the battery.
Some atrial leads have passive fixation elements that affix to the atrium over time. One example of a single pass lead is shown in U.S. Pat. No. 4,289,144 issued to Gilman on Sep. 15, 1981. Gilman relates to an atrial-ventricular pacing lead having outwardly extending tines surrounding the ventricular lead electrode and/or the outer atrial lead electrode. Another example is shown in U.S. Pat. No. 4,643,201 issued to Stokes on Feb. 17, 1987 which titled "Single-Pass A-V Lead." Stokes relates to a ventricular lead branch which has excess length adapted for a larger heart and an atrial lead branch having a "J" shape. A problem with these leads is that the electrodes are much more likely to be displaced from the wall of the atrium. When the electrodes are placed far from the wall, there can be detrimental effects. For example, the electrode may be unable to sense a tachycardia condition. Another example might be that signals for pacing may be ineffective. Additional power may have to be used to pace the heart thereby depleting energy from the battery of the pulse generator of the pacing system.
There is a real need for a single-pass endocardial pacing lead that has improved fixation to the wall of the atrium of the heart. A single-pass lead equipped with such an electrode would allow for better sensing capability and better pacing delivery to the heart. In addition, there is a need for a single-pass lead having an electrode for positioning within the atrium that allows for tissue ingrowth. Tissue ingrowth further enhances the electrical performance of the electrode. In addition, the lead and electrode is further stabilized within the heart as a result of tissue ingrowth.